[Server] Instrument command processing, game starts, and event loops

Add a lock-free MetricsRegistry that accumulates per-command processing
times in preallocated histogram slots (one per protobuf command type,
bucketed at 1/5/10/25/50/100/250/500/1000/2500/5000 ms +Inf). The
hot-path observeCommand() uses only relaxed atomic adds — no locks,
no allocations, no cache-line ping-pong beyond the unavoidable counter
updates.

Wire the registry into AbstractServerSocketInterface::processCommandContainer()
so every processed command is attributed with its container's wall-clock
time. When a container exceeds metrics/slow_command_ms (default 500),
a warning is logged including the connected username.

Add an EventLoopWatchdog heartbeat that runs on every socket pool thread.
If a heartbeat overshoots metrics/stall_warn_ms (default 2000 ms), the
overshoot is recorded in atomic counters and a warning is logged. Both
thresholds are configurable in servatrice.ini; setting stall_warn_ms to 0
disables the watchdogs entirely.

Track game-start durations via a separate histogram in MetricsRegistry.
Server_Game::startGameNow() measures the time from zone creation through
player materialization and reports it via Server::observeGameStartDurationMs().

Add a live card-count gauge: Server_Game exposes getCardsInGame() and
Servatrice::getCardsInGamesTotal() sums across all running games under
the appropriate read locks.

Include a standalone metrics_registry_test (Google Test) that validates
empty registries, single/multi-sample histograms, kind encoding,
overflow-slot collapse, negative-duration clamping, gauge rendering,
and the game-start histogram separation.

Took 10 minutes

servatrice/src/event_loop_watchdog.h

@@ -0,0 +1,50 @@
+/**
+ * @file event_loop_watchdog.h
+ * @ingroup Servatrice
+ */
+
+#ifndef EVENT_LOOP_WATCHDOG_H
+#define EVENT_LOOP_WATCHDOG_H
+
+#include <QElapsedTimer>
+#include <QObject>
+#include <QString>
+
+class Servatrice;
+class QTimer;
+
+/**
+ * @brief Detects blocked or overloaded worker event loops.
+ *
+ * One instance lives in each socket pool thread. A heartbeat timer tick that
+ * arrives late means the loop spent that time elsewhere: busy work, a queued
+ * slot, or a hard wedge. Overshoots past the configured threshold bump
+ * lock-free counters on the metrics registry and log one warning per stall,
+ * so a stuck pool thread becomes visible instead of silent lag.
+ */
+class EventLoopWatchdog : public QObject
+{
+    Q_OBJECT
+public:
+    /// How often the heartbeat expects to fire. Small enough to catch short stalls.
+    static constexpr int HeartbeatIntervalMs = 500;
+
+    EventLoopWatchdog(Servatrice *_servatrice, QString _threadName);
+
+    /**
+     * Starts the heartbeat timer. Must be invoked queued after the instance
+     * was moved to its target thread so the timer lives there too.
+     */
+    void start();
+
+private slots:
+    void checkHeartbeat();
+
+private:
+    Servatrice *servatrice;
+    QString threadName;
+    QElapsedTimer sinceLastTick;
+    QTimer *heartbeatTimer = nullptr;
+};
+
+#endif